I've got you covered, abricoetzer.

Creating a purchase order (PO) in QuickBooks Online is pretty easy. It's possible the PO has line items at the Account details level with no account selected. Or there are items in both levels, that's why you're getting an error. See the screenshot below for your visual reference.

You'd need to select an account if you enter the item in the Category details section. This will help identify your purchases. 

The following steps outlined below will guide you through creating a PO:

1. Click the New ⨁ button.
2. Select Purchase Order.
3. From the Supplier ▼ drop-down menu, select or add a supplier.
4. If you are shipping a product directly to a customer, click the Ship To ▼ drop-down menu and choose the customer.
5. If you're using custom fields, fill them in as needed.
6. In the Category details and/or Item details section, enter the purchases.
7. Click Save and close to complete the entry.

You can also refer to this article for more details: Create purchase orders in QuickBooks Online. It includes apply the PO to a transaction and other important information.

Should you need anything else, please let me know. I'm always around to help ensure your success.